This invention relates to ink jet printers and deals more particularly with an improved apparatus or circuit and related method for driving an ink jet printer head having an electrically energizable activating element such as a piezoelectric one.
In an ink jet printer a receiving surface on which a graphic is to be created is moved relative to one or more ink jet printer heads in a line scanning fashion. As each printer head moves along a scan line it moves past a succession of points on the line in relation to each of which the printer head may eject a drop which lands on and prints a dot at the position. In one type of printer head the head is actuated for each potential print point on the scan line, to eject a drop of ink for each such position, and then the drop is electrostatically controlled during its flight from the printer head to the receiving surface to either direct it onto the receiving surface or away from the receiving surface depending on whether the scan line point in question is to be printed or not. In such a printer head the actuation frequency, or the time between successive actuations, is dependent on the speed of the printer head along the scan line. That is, the actuation frequency, or the time between successive actuations, will change if changes are made in the speed of the printer head relative to the receiving surface.
In another type of printer head, referred to as a "drop-on-demand" printer head, as the printer head is moved along a scan line it is actuated to produce a drop of ink only for those potential print positions along the scan line onto which printing is wanted. Therefore, the amount of time elapsing between successive actuations is dependent not only on the speed of the printer head relative to the receiving surface but also on the pattern in accordance to which dots are to be printed along the scan line.
In either type of printer head described above, after a drop is ejected from the head it travels for some distance in free flight from the printer head to the receiving surface along a trajectory path dependent on the velocity at which the drop is ejected. Changes in the ejected velocity therefore change the location at which a drop strikes the receiving surface and are quite undesirable. Also, for good printing all ejected drops should be of substantially the same volume so that all dots printed on the receiving surface by the separate drops are of substantially consistent size.
Because of fluid and mechanical dynamics involved in the actuation of a printer head, including resonances and other phenomena, the ejected drop velocity, and also to some extent the drop volume, varies widely in many printer heads with changes in the actuation frequency or the time elapsing between successive actuations. This may be somewhat troublesome in the use of electrostatically deflected printer heads in cases where the printer head is moved at different speeds relative to the receiving surface. It is, however, particularly troublesome in the case of drop-on-demand printer heads in which the inherent operation of the printer involves a wide range in the elapsed time occurring between successive pulses. That is, while scanning a line during one portion of the line the printer head may be actuated to print a dot at every potential print point in which case a very short elapsed time occurs between successive actuations, and along other portions of the line the printer head may be actuated to print a dot only at some occasional potential print points in which case the time elapsing between successive actuations is considerably lengthened.
The purpose of the invention is therefore to provide a driving circuit for an ink jet printer head, particularly useful with drop-on-demand printer heads, but also useful with electrostatically deflected heads, for causing the printer head to eject drops at a constant velocity, and of substantially constant size, despite changes in the time elapsing between successive actuations.
A further object of the invention is to provide a circuit of the foregoing type which may be readily adjusted to suit the particular printer head with which it is to be used.
Other objects and advantages of the invention will be apparent from the following description and claims taken in conjunction with the accompanying drawings.